1. Field of the Invention
The instant invention is directed to a process for producing a polyacetal polymer. More specifically, the instant invention is directed to a process for producing a polyacetal polymer by reacting a polyacetal forming polymerization mixture in a reaction zone in which at least a pair of shafts each provided with a plurality of intermeshing elliptical shaped plates are provided. Still more specifically, the instant invention is directed to a process for producing a polyacetal copolymer in which a polyacetal copolymer forming polymerization mixture is reacted in a reaction zone in which the reactants are reacted in the presence of at least a pair of shafts each provided with a plurality of intermeshing elliptical shaped plates.
2. Background of the Prior Art
Polyacetal polymers represent a very important class of engineering resins which have been known for many years and are disclosed in, for example, Kern et al., Angewandt Chemie 73, pp. 176-186 (Mar. 21, 1961), and Sittig, M., Hydrocarbon Process and Petroleum Refiner 41, pp. 131-170 (1962). The importance of polyacetal resins as a commercial product is in no small part due to its unique combination of physical properties. The stiffness, toughness and inertness of polyacetal resins has resulted in widespread use in molded and extruded objects. Polyacetal polymers are formed by polymerization in the liquid phase, and in the presence of a suitable polymerization catalyst. During polymerization, the polymer forms as a solid or highly viscous polymeric mass substantially filling the entire volume of the reactor. Such a processing procedure is unattractive in view of the difficulty of maintaining proper temperature control of the solidified polymeric mass. In addition, further processing, such as washing or purification, requires that the solid polymer mass be ground, chopped or pulverized. This is costly and oftentimes commercially impractical in view of the extreme toughness of the polymer.
An alternate process for producing polyacetal polymers, which avoids the problems discussed above, is to dilute the polymerization mixture with a large amount of an inert solvent which prevents solidification of the reactant mass. Besides, the obvious problems associated with employment of large amounts of solvent, such as high cost, toxicity, explosion potential and the like, the use of large amounts of inert solvents results in lower molecular weight products as well as decreased polymerization rates. For this reason, the employment of significant quantities of solvent has not been looked upon as a variable alternative for successful production of polyacetal polymers.
To overcome the problems associated with highly viscous polyacetal products, without resorting to the use of large quantities of solvent, several processing schemes have been suggested in the prior art. Among these useful processing schemes in one suggested by Seddon and Russell in U.S. Pat. No. 3,253,818 issued on May 31, 1966. In this patent, a process is suggested which employs, as a reactor, a cylindrical barrel in which a screw member, coaxial with the barrel and provided with a plurality of interrupted threads causes reaction of the polymerization reactants by moving the polymerization mixture, disposed in the interruptions between the projections of the screw, over teeth which project from the inner surface of the barrel. This represents an improvement over the prior methods of production producing polyacetal polymers. However, the degree of mixing attained by this method still leaves something to be desired. As those skilled in the art are aware, unless good mixing is attained the reaction mixture does not adequately contact the cooling surface of the outer wall resulting in elevated reaction temperatures. Oftentimes, if the reaction temperature is too high, the monomer or monomers boil thus presenting the undesirable case wherein the monomer is in the gaseous state and the catalyst in the liquid state. Obviously, such a condition results in minimum contact. This, in turn, requires the introduction of greater quantities of catalysts, with the attendent cost and processing problems associated therewith. But, of even greater significance, it results in lower conversion rates. The conversion rate, of course, is the most critical parameter in polymerization processing.
Although the process discussed above represented a significant advance in the art, the problems associated with this processing scheme encouraged the development of other processing procedures which attempted to produce polyacetal polymers without significant employment of solvent. U.S. Pat. No. 3,442,866 issued on May 6, 1969 to Seddon and Scarbrough suggested the alternative processing scheme wherein a reactor was designed which comprised a pair of intermeshing parallel screw members disposed in an elongated case. The screw members had their threads running in the same direction and longitudinally rotated in that same direction. This scheme represents an improvement in terms of better mixing of the reactants to produce a lower reaction temperature. However, the design employed oftentimes does not produce sufficient grinding and/or pulverizing so that the original problems associated with a thick polymeric mass remain. That is manifested in the high horsepower requirements to move the polymeric mass through the reactor. Oftentimes, such a reactor "torques out". That is, the power supplied by the motor is insufficient to transport the polyacetal polymer product to the outlet of the reactor, plugging the reactor.
The above remarks suggest the need for a new process for producing polyacetal polymers which produce a finely divided product, produced without significant use of solvents, which may be conveyed through the reactor at temperatures lower enough to provide excellent conversion rates and low catalyst usage while providing sufficient grinding and pulverization so as to prevent the need for extraordinary power capacity.